A solid imaging process is one wherein photohardenable materials are polymerized layer by layer in an imagewise fashion. Generally the imaged region, that is that region exposed to actimic radiation, is required to harden to such an extent that it can withstand disturbing forces such as shear forces without significant distortion or damage. In most current liquid based solid imaging systems, for example the process described in U.S. Pat. No. 5,006,364 to Fan; a doctor blade is used to distribute and smooth the photohardenable liquid on top of a platform, a support structure, or a previously imaged region. If the image region is insufficiently hardened or has poor initial green strength; it may be easily distorted by the force and movement of the doctor blade. Unsupported imaged regions, for example cantilevered layers and bridging layers between supports or other parts of an imaged article are of particular concern since these regions represent outer edges of the imaged article, and any deformation of these layer may result in a loss in tolerance of the article.
Epoxy based photohardenable compositions are capable of producing articles having exceptionally high modulus; however, the modulus usually takes longer to develop after exposure when compared with, for example, acrylate photohardenable compositions. It is possible to speed-up the modulus development of epoxy photohardenable compositions by mixing into the epoxy composition free radical polymerizable compounds such as acrylate or vinyl compounds; but if the acrylic matrix causes the imaged region to vitrify too quickly, the imaged layer formed from such a mixed resin composition may actually have lower initial green strength and possibly even lower final part strength than that of a layer or complete article formed from the more slowly curing epoxy only composition. It has been found that only certain formulations of mixed free radical and cationically polymerizable monomers compositions provide proper control of the rate and degree of cross-linking short of vitrification and therefore diffusion within the exposed region of the photohardenable composition and lead to an optimum balance which affords high initial green strength, high article green strength, and high article final strength.
An object of the present invention is to provide a precisely formulated and blended mix of epoxy and acrylic monomers, prepolymers and oligomers to control the diffusion of the activated photoinitiators such that these activated species are concentrated in the imaged region increasing the control of polymerization reaction so as to improve initial green strength and image resolution. The precise formulation relates to the proper selection and concentration of the epoxy and acrylic species that are blended into the photohardenable composition.
Another objective of the present invention is to control the initiation of polymerization throughout the exposed region and thus control the depth of exposure by selection and formulation of the photoinitiators to provide activity at at least two of the wavelengths of activation radiation provided by the radiation source.
Another objective of the present invention is to provide a photohardenable composition optimized for use with the argon ion laser.
Another objective of the present invention is to provide a photohardenable composition that is tolerant to the presence of water and to provide a composition that shows little or no variation in initial green strength of imaged regions or of cure rate as a result of the relative humidities of work areas that are 50% or even higher.
Another object of the present invention is to provide a method by which an article having high initial green strength and high image resolution can be produced by solid imaging techniques.